The present invention relates to an induction motor and, more particularly, to a highly precise variable speed servo induction motor operable in a wide speed range.
Conventional induction motors have been used as low-cost, roughly constant-speed motors. Along with recent developments of drive control techniques supported by developments in electronic devices and in software, induction motors have been used as highly precise variable speed servo motors. Servo motors must be smoothly operated in a low-speed range including zero speed. For this purpose, desired torque must be generated. Fidelity of desired torque generation is represented by the fact that a generated torque TG is substantially equal to a constant (without variations) load torque TL; that is, a torque ripple .DELTA.T (i.e., harmonic torque) comes to minimum for TG=TL+.DELTA.T when the induction motor is operated to generate the load torque TL.
The performance of the induction motor from the viewpoint of an object to be driven is determined by the induction motor itself and a variable frequency power source (to be referred to as a driver hereinafter) for applying electrical energy to the induction motor. The frequency of the variable frequency power source is called a primary frequency since the energy is applied to the primary winding of the induction motor. This frequency is represented by f1. Most of the conventional drivers generate simple 3-phase rectangular wave voltages. According to the influences of a magnetomotive force caused by space and time harmonic components, since the voltage waveform includes time harmonic components which are (6k.+-.1) (k=1, 2, 3, . . . ) times the primary frequency f1, a torque ripple component having a frequency of 6kf1 appears in a "force wave" proportional to the output torque TG of the induction motor. In other words, such a torque ripple is regarded as caused by the driver. Recent drivers have been improved due to developments of electronic devices (e.g., LSIs and power-controlled semiconductors); current, speed and position detectors; and software techniques for achieving highly precise, high-speed data processing. A electric current having substantially a sinusoidal wave can be supplied from the driver to a induction motor in the wide, variable frequency range.
However, when the induction motor serves as a servo motor driven at a very low speed below the frequency f1 (typically below several Hz), the torque ripple of the 6kf1 harmonic component is not generated by the driver but by the induction motor. Therefore, it is indispensable to eliminate the torque ripple generated by the induction motor itself.